The present invention relates to an optical recording apparatus that can record data by applying light to a recording medium. More particularly, the invention relates to an optical recording apparatus which can record data signals at high density, which has an objective lens of a large numerical aperture and which is designed to record data signals with the objective lens held at a short distance from the surface of an optical recording medium. The invention also relates to an optical recording/reproducing method in which data is recorded on and reproduced from an optical recording medium, while a specific distance is maintained between an objective lens of a large numerical aperture and the optical recording medium.
A representative technique of recording and reproducing data by using near-field light is to use two lenses (hereinafter referred to as xe2x80x9ctwo-group lensxe2x80x9d). One of these lenses is a solid-immersion lens (SIL). The other of the lenses is a condensing lens. This technique has been devised to make a spot diameter even smaller, thereby to meet the demand that data be recorded at high density on optical disks. The SIL is interposed between the condensing lens and an optical disk and provides numerical aperture larger than that of the condensing lens. The SIL is a high-refractivity lens made by cutting a part of a spherical lens. It is positioned, with its spherical surface opposing the condensing lens and its other surface facing the optical recording medium.
A near-field signal output by the SIL may be used to record data and reproduce data optically. To this end, two requirements must be fulfilled. First, the surface of the SIL and the optical recording medium must be reduced to a value (half the wavelength of the light or a shorter distance, or 200 nm or less in most cases) that serves to generate a near-field light beam and be maintained at this value. Second, focus control must be achieved to set the spot of a condensed beam at a constant value on the optical recording medium.
A representative technique employed in optical recording apparatuses of this type is to use an air-bearing slider. A two-group lens is mounted on the air-bearing slider and the optical recording medium is rotated. A film of air is thereby formed between the optical recording medium and the two-group lens mounted on the bearing. The two-group lens floats by virtue of the pressure of the film, maintaining a constant distance between the surface of the SIL and the optical recording medium. This technique is similar to the method used in magnetic recording to control the distance between the magnetic head and the recording medium.
A lubricant may be applied to that surface of the optical recording medium that receives light, thereby increasing the focal depth. In this case, the focus can be stabilized even if the optical recording medium has depressions and projections at the surface or dusts exists on the surface of the medium in the distance control achieved by the use of the above-mentioned air-bearing slider.
These conventional techniques are disadvantageous, however. First, in the method of using the air-bearing slider only, no active control is carried out against the external disturbance resulting from scars or depressions existing in the surface of the optical recording medium, causing the possibility for defocusing. Further, the gap cannot be controlled unless the medium is rotated, making it impossible to accomplish focus control.
Secondly, in the method of applying a lubricant to the light-receiving surface of the optical recording medium, defocusing may occur place even if the focal depth is increased, as long as the optical medium has depressions and projections at the surface or dusts exists on the surface of the medium. Moreover, this method is inappropriate for disks having a photoresist coating, such as CDs and DVDs, because a lubricant is applied to the light-receiving surface of the optical recording medium.
The present invention has been made in view of the foregoing. The object of this invention is to provide an optical recording apparatus in which the distance between the surface of SIL and the optical recording medium can be maintained constant to prevent defocusing, whether the disk is rotating or not, even if scars or depressions exist in the surface of the medium, and which can record data at high density, and which can be manufactured with ease. Another object of the invention is to provide an optical recording/reproducing method that can record data at high density.
To achieve the first object, an optical recording apparatus according to the invention comprises: first optical means for condensing light to be applied to an optical recording medium, said first optical means having a numerical aperture; second optical means interposed between the first optical means and the optical recording medium, for attaining a numerical aperture larger than the numerical aperture of the first optical means; and main control means for maintaining a predetermined distance between the second optical means and the optical recording medium, by utilizing a linear characteristic of an amount of light reflected from the optical recording medium in a near-field region provided between an end of the second optical means and the optical recording medium.
The optical recording apparatus may further comprises means for setting a threshold value corresponding to the amount of the light reflected from the optical recording medium and a control target value falling within the linear characteristic of the amount of light reflected, and a sub-control means for suppressing the influence of the amount of light reflected when the amount of light reflected is greater than the threshold value. In this case, the main control means or the sub-control means, or both are used in accordance with a relation the amount of light reflected has with the threshold value or the control target value.
To attain the second object, an optical recording/reproducing method according to the invention comprises: a main control step of maintaining a predetermined distance between second optical means and an optical recording medium by utilizing a linear characteristic of an amount of light reflected from the optical recording medium in a near-field region provided between an end of the second optical means and the optical recording medium, thereby to impart to the second optical means a numerical aperture larger than the numerical aperture of a first optical means, said second optical means having been interposed between the first optical means and the optical recording medium; a setting step of setting a threshold value for the amount of light reflected from the optical recording medium and a control target value falling within the within the linear characteristic of the amount of light reflected; and a sub-control step of suppressing the influence of the amount of light reflected when the amount of light reflected is greater than the threshold value. In the method, the main control step or the sub-control step, or both are performed in accordance with a relation the amount of light reflected has with the threshold value or the control target value.
In the optical recording apparatus according to the present invention, a constant distance can be maintained between the objective lens and the optical recording medium, no matter whether the medium is rotating or not, while a near-field beam is emerging from the optical recording medium. The size of the light spot formed on the surface of the medium can thereby remain constant. It is therefore possible to record data on and reproduce data from optical disks by the use of a near-field light beam. In addition, stable focus control can be achieved, in spite of external disturbances.
Moreover, the present invention makes it possible to record data by the use of a near-field light beam in cutting machines that are designed to manufacture DCs and DVDs.